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Registered Member #859
Joined: Mon Jun 25 2007, 10:23PM
Location:
Posts: 11
Ive been buiding my first coilgun using a boost converter for a couple days now. Ive finished most of everything but im having a problem with the SCR conducting continuously after the capacitor bank has been discharged.
Schematic
I think the problem is that the capacitors keep charging to 15v through the inductor and keeping the scr open but I cant find a way to prevent it. I also dont think I connected the SCR in the best way.
Banned Registered Member #110
Joined: Fri Feb 10 2006, 12:23AM
Location: Banned City
Posts: 85
I thought about not replying because Rocketry76, in a single sentence, thoroughly responded with the reason and resolution. One of another method is to use a High-Side IGBT or Mosfet switch above L1. This is far more involved than a relay but increases the elegance, no bulk, no relay sound. The drawback is that the addition of switches in series with the L1, Q1 circuit will increase power wasted. Example, if your Q1 is consuming 10 watts from a 100 watt supply, you hand 90 possible watts to deliver to capacitors. Adding another switch will suck up another 10 watts which inturn steals from the caps with a lower 80 watts of power. This is good for small sized low power chargers installed in small areas.
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
If you have a high power IGBT use it dyreclty for coil switching. No sense to use a semiconductor switch on high side, this increases design complexity. You can use it on low side though. In my multistage design i used 5 floating SCR's and a single IGBT switch grounding their cathodes. The configuration is like that:
The drive stage is connected to the sensors dyrectly (5V ttl) White the IGBT is driven by a 24V driver controlled by the microcontroller. The drive is on while one of the sensors triggers the RB0/int interrupt pulsing the correct pulsetime.
In your case a single scr and a singe igbt is sufficient. Just make sure it is driven properly.
Also you can simply switch off the charger pulling down the switch drive with a small bjt or an mosfet or pulling up the voltage feedback if you are using a controlled (es a TL494 boost converter) SMPS
Another option use the scr V-switch using a small sensing coil , this will improve efficiency too but it must be carefully designed
SCRs are funny devices but they are hard to turn off..
Registered Member #859
Joined: Mon Jun 25 2007, 10:23PM
Location:
Posts: 11
Thanks a lot for the suggestions, but I have a couple more questions.
What exactly does it mean when you say floating scrs?... Is the driver connected to the gate of the scr whos anode is connected to the cap bank and kathode is connected to the IGBTs collector? Will the coil be after the SCR, or am i completely off?
Also, my electronic jargon isnt quite complete yet... what exactly does high-side/low-side mean?
And two more things:
I believe I understand how a V-switch works, im just wondering how exactly one calculated where to wind the secondary coil? Does it have something to do with the projectile reaching a certain point?
And I've also read about a lot of your projects and I was wondering if you knew where I could go to find out about fullbridge/halfbridge/diagonal bridge designs and operation principles, all the searches on this forum only brings up tesla coil stuff.
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Need help troubleshooting first coilgun
